Temporary tables in SQL Server vs. table variables

Once you've written your T-SQL code, it's time to execute it. You'll most likely need a table in which to store data temporarily. SQL Server database architect Denny Cherry explains your four options while offering the pros and cons of using temporary tables vs. table variables in SQL Server.

When writing T-SQL code, you often need a table in which to store data temporarily when it comes time to execute that code. You have four table options: normal tables, local temporary tables, global temporary tables and table variables. I'll discuss the differences between using temporary tables in SQL Server versus table variables. Each of the four table options has its own purpose and use, and each has its benefits and issues:

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Local temporary tables are temporary tables that are available only to the session that created them. These tables are automatically destroyed at the termination of the procedure or session that created them.

Global temporary tables are temporary tables that are available to all sessions and all users. They are dropped automatically when the last session using the temporary table has completed. Both local temporary tables and global temporary tables are physical tables created within the tempdb database.

Table variables are stored within memory but are laid out like a table. Table variables are partially stored on disk and partially stored in memory. It's a common misconception that table variables are stored only in memory. Because they are partially stored in memory, the access time for a table variable can be faster than the time it takes to access a temporary table.

Creating indexes on SQL Server tables

Because both local and global temporary tables are physical tables within the tempdb database, indexes can be created on these tables to increase performance as needed. As with any index creation, this process can take time on larger tables. Because temp tables are physical tables, you can also create a primary key on them via the CREATE TABLE command or via the ALTER TABLE command. You can use the ALTER TABLE command to add any defaults, new columns, or constraints that you need to within your code.

Unlike local and global temporary tables, table variables cannot have indexes created on them. The exception is that table variables can have a primary key defined upon creation using the DECLARE @variable TABLE command. This will then create a clustered or non-clustered index on the table variable. The CREATE INDEX command does not recognize table variables. Therefore, the only index available to you is the index that accompanies the primary key and is created upon table variable declaration.

More on SQL Server tables:

There are limitations on how you can use table variables. The limitations began changing with SQL Server 2005. Namely, table variables could not be used as the destination of an INSERT EXEC command such as

insert @variable exec sp_who

Starting in SQL Server 2005, this limitation was removed and table variables can now be used as the destination for INSERT EXEC commands.

Changes to table variables in SQL Server 2008

Microsoft SQL Server 2008 introduces another important change to table variables. The latest SQL Server edition allows you to create a table variable as an input or output parameter of a stored procedure. An array of information can now be passed to a stored procedure without creating a string or XML block and then having to parse it out after executing the stored procedure. There are some restrictions on this however. The table variable parameter must be declared based on a table data type, and the variable must be read-only. When compared to not having this ability at all, these restrictions are minor inconveniences at best.

In the below sample code, a user-defined table type is created and a stored procedure is then defined using this table type. A local variable is declared also using the table type, data is loaded into the variable and then the array of data is passed from the local variable to the input parameter where the records are used within the stored procedure. Here is an example:

How do the internal workings of SQL Server perform differently between table variables and temporary tables?

The differences between accessing tables and variables cause the internal processes within SQL Server to treat the objects quite differently. Temporary tables are actually physical tables, so the SQL Optimizer and locking engine handle the tables just as they would any other database tables. Because reads to a temporary table are made (including local temporary tables), a read lock is placed on the table.

This locking process takes time and CPU resources. When reading from a table variable – because the table variable is stored partially within memory and cannot be accessed by any other user or process on the system – SQL Server knows locking is not required. In a very busy database, this lack of locking can improve system performance because locks do not have to be taken, escalated and checked for each data access operation.

Limits of temporary tables and table variables

Temporary tables and table variables both have their strengths, but they both have weaknesses too. On a heavy load system that has lots of usage of temporary tables, the disk array servicing the tempdb database will experience a higher than expected load. This happens because all reads and writes to the temporary tables are done within the tempdb database. Table variables will perform poorly with large record sets, especially when doing joins because there can be no indexes other than a primary key. Beware, though, when many users start using table variables -- large amounts of RAM are used because all temporary tables are stored and processed directly in memory. Table variables should hold no more than 2 Megs to 3 Megs of data each (depending on user load and system memory).

Both temporary tables and table variables can be extremely useful tools in developers' and administrators' arsenals; however, care must be taken as to when to use each solution. There is no end-all solution, and you must choose the correct solution for the correct situation.

All information provided about Microsoft SQL Server 2008 (Katmai) is based on beta edition 10.0.1019 of the software and is subject to change without notice.

ABOUT THE AUTHOR

Denny Cherry is a DBA and database architect managing one of the largest SQL Server installations in the world, supporting more than 175 million users. Denny's primary areas of expertise are system architecture, performance tuning, replication and troubleshooting.

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